<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>过渡练习-进度条</title>
    <style>
        .bar{
            width: 150px;
            height: 15px;
            border: 1px solid #f00;
            border-radius: 7px;
            padding:1px;
        }

        .bar_in{
            width: 50%;
            height: 100%;
            background-color: red;
            /* 谁做过渡给谁加 */
            transition: all 1s;
        }

        .bar:hover .bar_in {
            width: 100%;
        }
    </style>
</head>
<body>
    <div class="bar" >
        <div class="bar_in" ></div>
    </div>
</body>
</html>